1. Field of the Invention
The present invention relates to a printer and printer head, and more particularly, to a line printer using an ink-jet method.
2. Description of the Related Art
Conventionally, ink-jet printers have been designed to print a desired image, characters and so on by ejecting ink droplets onto the paper selectively by the nozzles placed sequentially in a direction substantially perpendicular to the paper-feed direction.
In such a line printer, a thermal-method printer for example, as disclosed in Japanese Unexamined Patent Application Publication No. 2001-71495, head chips are used to form a printer head in order to improve yield and to avoid influence of wiring resistance. In this case, the head chip is formed such that ink is held in ink bed, and is heated by a heater to eject ink droplets from the nozzles. The head chip is made of one semiconductor substrate on which heaters and so on are created for a plurality of such nozzles. In a printer, a plurality of such head chips are placed to form a line head, for example, thereby making it possible to simplify the total configuration.
However, a printer head consisting of such head chips in a conventional fabrication has various problems described below, thereby having defects for full practical use.
A first problem among the above will be described below, which is to be solved by the present invention. This problem is that, for these head chips, irregularity of the characteristics cannot be prevented. If the irregularity is large, as shown in FIG. 29, on the boundary of the ranges that are covered by the head chips 1A and 1B, which are placed adjacently, there appears dot-density difference of the printout between the ranges covered by the head chips 1A and 1B, respectively. Thus, for example, when printing a background in a single color, a phenomenon occurs that on the boundary part, there appear vertical stripes in the direction of printout, deteriorating quality of the print result.
Next, a second problem with a printer head in a conventional fabrication will be described below, which is to be solved by the present invention. In this connection, deterioration of print result when using an ink-jet printer is caused not only by the above-mentioned irregularities of head chips, but also by positioning errors of head chips.
For an ink-jet printer, high quality print result is required, so that printing has been performed using small droplets with high density in these years. However, when using the same color ink-droplets, if deviation of an impact point from a target point is more than a half of a dot diameter, deterioration of print quality is detected. This means that nozzles need to be positioned with high precision, otherwise a positioning error affects an impact point of an ink droplet. In this connection, when a dot 40 μm in diameter is created by adhering an ink droplet onto a paper to secure print result of 600 dpi, a positioning error of a dot is allowed by not more than 20 μm for the same color ink-droplets.
Therefore, in a printer, even if irregularity of the characteristics of head chips is prevented for practical use, when aligning a plurality of head chips to form a printer head in a conventional fabrication, head chips need to be aligned with very high precision, which is practically difficult, thereby making it inevitable to deteriorate print result.
Specifically, as shown in FIG. 30, for example, when placing the head chips 30A to 30D in alignment to form a printer head, the head chips 30A to 30D are usually placed in alternate shift in the direction of feeding paper, thereby making it possible to place the nozzles at regular intervals in the direction of alignment of the head chips 30A to 30D. In such placement of head chips, misplacement may occur as shown in FIGS. 31, 32 and 33.
In the case as shown in FIG. 31, the head chip 30C is misplaced in the direction of alignment of head chips. In this case, at the boundaries of the misplaced head chip 30C and the adjacent head chips 30B and 30D, nozzle pitches are not uniform, thereby resulting in print irregularity with stripes in the direction of paper feed in the same manner as described above as in FIG. 29.
In the case as shown in FIG. 32, the head chip 30B is misplaced in the direction of paper feed. In this case, the nozzles of the head chip 30B are misplaced just as the misplacement of the head chip in the direction of paper feed, thereby resulting in a situation that, for example, when printing a straight line in a lateral direction, a step-wise output will be produced.
In the case as shown in FIG. 33, the head chip 30D is placed inclining, thereby resulting in a situation that, for example, when printing a straight line in lateral direction, a bended line will be produced. Moreover, as viewed from the paper feed direction, the nozzle pitches of the head chip 30D are shorter due to the inclination of the head chip, more particularly, the nozzle pitch suddenly changes at the boundary of the adjacent head chip 30C, thereby resulting in deterioration of uniformity of the print result. In this connection, deterioration of the print result is significant when printing a line image in the paper feed direction.
Additionally, quality deterioration of print result due to positioning error of head chips occurs when printing in different colors, which is detected as deterioration of registration and reproducibility in color.
Normally, when printing an image in color, ink droplets of yellow, magenta, cyan and black are adhered onto a paper. Corresponding to this, as shown FIGS. 34 to 36, for a printer head printing an image in color, alignment of head chips is formed for each color, that is, Y for yellow, M for magenta, C for cyan and B for black. In FIGS. 34 to 36, arrays of the head chips 30A to 30D shown in FIGS. 30 to 33 are indicated as rectangular forms for simplicity.
When placing these arrays of head chips, misplacement may occur as shown in FIGS. 34 to 36. FIG. 34 shows the case where an array of particular color head chips is misplaced in the direction of the head chip arrays. In this case, the image of the color assigned to the misplaced head chip array (M for magenta in the case of FIG. 34) will be printed with some displacement in lateral direction compared with the image of other colors.
In the case as shown in FIG. 35, a head-chip array of particular color (the head chip array of Y for yellow in this case) is misplaced in the direction of placing head chip arrays. In this case, the image of the color assigned to a misplaced head chip array will be printed with some displacement in the paper feed direction compared with the image of other colors.
In the case as shown in FIG. 36, an array of particular color head chips is inclined (the head chip arrays M for magenta, C for cyan and K for black in this case). In this case, the image of the color assigned to the inclined head chip array will be printed as twisted compared with the image of other colors.
In this connection, misplacement of dots in different colors is not so severely observed as in the case of the same color, but will be detected if displacement is approximately 100 μm.